At present, color mixing concept is widely used to obtain white light with expected CCT (correlative color temperature) and CRI (color rendering index). At the same time, it demands a higher requirement on electronic driver design. The electronic driver should be able to drive multiple LED strings. However, the electronic driver driving multiple LED strings should have good response to a dynamic load. In the prior art, the popular peak current control buck topology circuit is a good option for driving multiple strings because of its good response to dynamic voltage variation.
FIG. 1 is a typical fixed frequency peak current control buck topology circuit used for driving multiple strings. The relation between an output voltage and a current flowing through the strings may be obtained via the following formulas.
                              D          =                                    V              out                                      V              in                                      ,                            Formula        ⁢                                  ⁢                  (          1          )                    wherein D is a duty cycle of a control signal, Vout is an output voltage of the strings, and Vin is an input voltage;
                                          Δ            ⁢                                                  ⁢            I                    =                                                    (                                                      V                    in                                    -                                      V                    out                                                  )                            ·              D                                                      F                s                            ·              L                                      ,                            Formula        ⁢                                  ⁢                  (          2          )                    wherein ΔI is a ripple current on an inductor L1, Fs is a control signal, and Ipk is a controlled peak current flowing through the inductor L1;
                                          I            o                    =                                    I              pk                        -                                          1                2                            ⁢              Δ              ⁢                                                          ⁢              I                                      ,                            Formula        ⁢                                  ⁢                  (          3          )                    wherein Io is a current flowing through the strings. Formula (4) Io=K1(Vout2−Vin·Vout)+Ipk can be derived from Formulas (1), (2) and (3), wherein
      K    ⁢                  ⁢    1    =            1              2        ⁢                  (                                    F              s                        ·            L            ·                          V              in                                )                      .  A current-voltage chart shown in FIG. 2 can be easily obtained from Formula (4). As can be seen from FIG. 2, when this circuit is used to driver multiple strings, the current flowing through the strings also changes dynamically when the output voltage changes.
FIG. 3 shows the problems above by waveform. Assume that a transistor Q2 in the circuit shown in FIG. 1 is always turned off and a duty cycle of a converter is set to be 50%. At t1 period, a transistor Q3 is turned off, then the output voltage Vout is a sum of the whole three strings, i.e., Vout=V_str1+V_str2+V_str3, and at t2 period, the transistor Q3 is turned on, then the output voltage Vout is Vout=V_str1+V_str3. During the period when transistor Q3 is turned on, the output voltage Vout decreases, which causes the ripple current ΔI to decrease. As the controlled peak current Ipk flowing through inductor L1 always keeps constant by a current control loop, the current Io, flowing through the strings increases according to Formula (2), while the increased current Io is undesired.